Add rack middleware for proofing HTML at runtime

[DanielHeath]

Sample usage in a Rails app: Add to config/application.rb

config.middleware.use HTMLProofer::Middleware if Rails.env.test?
config.middleware.use HTMLProofer::Middleware if Rails.env.development?

This will raise an error at runtime if your HTML is invalid.

Particularly helpful for projects which have extensive CI.

[codecov-io]

Codecov Report

Merging #512 into master will increase coverage by 0.04%.
The diff coverage is 98.11%.

@@            Coverage Diff             @@
##           master     #512      +/-   ##
==========================================
+ Coverage   98.33%   98.38%   +0.04%     
==========================================
  Files          28       30       +2     
  Lines        1867     1918      +51     
==========================================
+ Hits         1836     1887      +51     
  Misses         31       31

Impacted Files	Coverage Δ
spec/html-proofer/middleware_spec.rb	100% <100%> (ø)
lib/html-proofer/runner.rb	99.05% <100%> (ø)	⬆️
lib/html-proofer/middleware.rb	96.66% <96.66%> (ø)
lib/html-proofer/log.rb	95.83% <0%> (+4.16%)	⬆️

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[DanielHeath]

@gjtorikian I think that was everything - anything else needed before a merge?

[gjtorikian]

Thank you!